Matrix Assisted Laser Desorption and Ionization (MALDI) Mass spectrometry of biomolecular ions was first demonstrated in parallel efforts by Tanaka using small metal particles suspended in glycerol and by Karas and Hillenkamp using organic matrices. In both cases the matrix performs the dual function of both adsorbing the laser light and ionizing the non-light absorbing biomolecule through specific yet poorly understood chemical reactions and physical desorption processes.
The MALDI technique is also applied for tissue imaging as the ability of mapping the distribution of targeted compounds in tissue is crucial in the field of human health (disease diagnostics, drug response). Caprioli has pioneered proteomics of intact tissue samples using a new imaging MALDI instrument. Only protein and peptide molecular ions above 5 kDa are imaged to 20 μm spatial resolution across the tissue surface. Pattern analysis of peptides expressed from tumor and non-tumorous tissue reveal strong correlations between numerous marker proteins/peptides and the disease state.
However, this technique has two major limitations. One is the difficulty to identify molecular ions below 5 kDa and to measure the concentration of low molecular weight drugs because of mass spectral congestion from isobaric lipids, oligosaccarides, nucleotides, and matrix ions. The second limitation is the discrimination of the detection to water-soluble molecules since the technique is based on the solvent-extraction which occurs during the addition of organic matrix solution to the tissue surface.
Alternatively, subcellular isotopic imaging by dynamic SIMS ion microscopy on freeze-fracture samples has also been developed for tissue analysis but it is limited to elemental and small molecule analysis.
Cluster ion beams are emerging as a powerful tool for the modifications of (surface cleaning/smoothing, very shallow implantation) and for SIMS analysis of surfaces. At typical cluster kinetic energies of a few tens of keV, each atom carries a very low energy minimizing damage. In contrast with monoatomic ion beams, higher density energy is deposited in the surface region with cluster ion beams yielding shallower implantation and minimizing channeling. In the analytical field, in recent years, the capabilities of SIMS have been greatly enhanced by the use of small cluster ions as projectiles.
The prior art lacks a method that allows the mass spectrometric identification of the molecular composition of surface or of a narrow subsurface region of organic solids or biomolecular tissues. We introduce a cluster ion bombardment method which when combined with laser ablation removes the topmost layer of such a solid in a way that causes very little damage to underlying layers of tissue material in the area of bombardment. In this way, the surface or near subsurface region can be sequentially interrogated by repeated steps of implantation and laser ablation to yield a spatial or volume distribution of molecules and elements within a solid sample which may be a biological tissue. It would also be desirable to further couple such a method to specialized and highly sensitive and selective mass spectrometric platforms in order to increase selectivity and minimize interferences in a complex sample such as tissue. Furthermore, it would be desirable to focus the cluster source to a submicron particle size so that certain regions of the sample (such as organelles) could be selectively implanted and subsequently interrogated with the laser.